With the use of terminals such as smartphones, an average amount of data used by mobile communication users is exponentially increasing, and users' demands for higher data transmission rates are continuously increasing. Generally, a high data transmission rate may be provided by providing communication using a wider frequency band or by improving frequency use efficiency.
However, it is quite difficult to provide a higher average data transmission rate with the latter method, because current-generation communication techniques have already provided frequency use efficiency close to a theoretical limit and thus increasing the frequency usage efficiency beyond the theoretical limit may not be easily achieved by technical improvement. Consequently, a practically probable method for improving a data transmission rate is to provide a data service in a wider frequency band. In this case, an available frequency band needs to be considered.
According to current frequency distribution policies, a wideband communication-possible band over 1 GHz is limited, and a practically selectable frequency band is only a millimeter (mmW) band over 30 GHz. In this high frequency band, unlike in a 2 GHz-band used by conventional cellular systems, signal attenuation with respect to a distance occurs excessively. Due to signal attenuation, for a base station using the same power as a conventional cellular system, service coverage is significantly reduced. To solve a corresponding problem, a beamforming scheme is widely used in which transmission/reception power is concentrated on a small space to improve transmission/reception efficiency of an antenna.
FIG. 1 illustrates a terminal and a base station that provides beamforming using an array antenna.
Referring to FIG. 1, a base station 110 transmits data while changing a direction of a downlink transmission (Tx) beam 111 with the use of a plurality of array antennas Array0 and Array1 for each of cells (or sectors) 101, 103, and 105. A terminal 130 receives data while changing a direction of a reception (Rx) beam 131.
In a system that performs communication by using the beamforming method, the base station 110 and the terminal 130 provide a data service by selecting a direction of a transmission beam and a direction of a reception beam that provide an optimal channel environment from among multiple transmission beam directions and reception beam directions. This process is equally applied to an uplink channel for transmitting data from the terminal 130 to the base station 110 as well as a downlink channel for transmitting data from the base station 110 to the terminal 130.
FIG. 2 illustrates an example in which the base station 110 transmits a signal through a transmission beam having a particular beam width in a system that performs communication using the beamforming scheme.
In FIG. 2, the base station 110 is installed in a position having a predetermined height 201 from the ground, and has a predefined beam width 203. The beam width 203 of the base station 110 may be defined with respect to an elevation angle and an azimuth. In FIG. 2, a transmission beam of the base station 110 is transmitted in a direction corresponding to an elevation angle 205. Although not shown in FIG. 2, the azimuth angle may be understood as a horizontal angle at which the transmission beam propagates.
FIG. 3 shows an example of the number of transmission beams the base station 110 may transmit and the number of reception beams received by the terminal 130 when, in the base station 110 installed in the same manner as in FIG. 2, the height 201 at which the base station 110 is installed is 35 m and the base station 110 transmits a transmission beam having a beam width of 5° with respect to an elevation angle and an azimuth in one sector, for example, having an angle of 30° and a coverage of 200 m. In the example of FIG. 3, ninety six (96) transmission beams having a beam width of 5° with respect to an elevation angle and an azimuth are used to configure one sector having an angle of 30° and a coverage of 200 m.
In a beamforming system, it is difficult for a terminal to form a large number of transmission/reception beams having a narrow beam width like a base station due to limitations in a physical space, performance, and cost. In the example of FIG. 3, the terminal 130 forms four (4) reception beams RX1, RX2, RX3, and RX4 to receive a transmission beam transmitted by the base station 110. In this case, an elevation angle beam width of a reception beam is about 90°.
In the beamforming system, generally, a narrow transmission beam has a high antenna gain, but due to a narrow beam width, communication performance may not be guaranteed if a direction of a transmission beam and a direction of a reception beam deviate from each other. Moreover, due to a limited transmission/reception range, communication may be instantly interrupted if a reflective object or an object through which the beam may not pass is disposed between the transmission beam and the reception beam. This problem is generally defined as a link fragility problem. A widely used method for solving this link fragility problem is that one terminal maintains a data transmission/reception channel with a plurality of base stations.
FIG. 4A illustrates an example in which a plurality of base stations maintain a data transmission/reception channel with a terminal in a general beamforming system.
Referring to FIG. 4A, a terminal 421 groups one or more nearby base stations 411 (Cell-0), 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11) into one serving base station group or cloud cell based on reception signal strengths, and periodically measures signals of the base stations 411, 413, 415, and 417 included in the group to maintain a data transmission/reception channel.
In the example of FIG. 4A, from among the nearby base stations 411, 413, 415, and 417, the beamforming system selects the base station 411 (Cell-0) having the highest signal strength as a serving base station of the terminal 421, and classifies the other base stations 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11) as scheduling candidate base stations. The terminal 421 transmits and receives a control signal and data through the base station 411 (Cell-0) in a normal channel condition, and at the same time, periodically measures signals of the base stations 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11), which are scheduling candidate base stations, to maintain a data transmission/reception channel with the base stations 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11). In this way, in the beamforming system, the terminal 421 may continue data transmission and reception through the other base stations 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11), included in the serving base station group, if a link between the terminal 421 and the base station 411 (Cell-0) is unstable.
In the beamforming system, the terminal generally generates a limited number of transmission/reception beams at a particular transmission/reception instant or time due to limitations in a physical space, performance, and cost. The example of FIG. 4A assumes that the terminal 421 forms one reception beam at every transmission/reception time. The terminal 421 needs to receive scheduling information for data transmission/reception from the base stations 411 (Cell-0), 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11) included in the serving base station group.
When two or more base stations of the serving base station group transmit scheduling information to the terminal 421, the terminal 421 needs to receive scheduling information, transmitted by the base stations 411 (Cell-0), 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11), at different times by using one reception beam. To this end, the scheduling information transmission times of the base stations 411 (Cell-0), 413 (Cell-4), 415 (Cell-5), and 417 (Cell-11) included in the serving base station group need to be defined differently, and the terminal 421 needs to determine whether each base station has transmitted scheduling information at each time. However, defining scheduling times of base stations in a way to avoid collision is generally difficult to achieve, and when the terminal 421 receives scheduling information from each base station included in the serving base station group, power consumption of the terminal 421 increases largely. To solve this problem a particular base station included in the serving base station group is designated as a base station for transmitting and receiving a scheduling and/or control signal for a corresponding terminal. In the example of FIG. 4A, the base station 411 (Cell-0), which is a serving base station, transmits and receives a scheduling and/or control signal for the terminal 421.
FIG. 4B illustrates an example in which a terminal receives a data packet by using a Hybrid Automatic Retransmit Request (HARQ) in a general beamforming system.
Referring to (A) of FIG. 4B, the terminal 421 receives scheduling information from the serving base station 411 (Cell-0), for example, in a subframe 0, to receive a data packet transmitted from the serving base station 411 (Cell-0) indicated by the scheduling information in the subframe.
Referring to (B) of FIG. 4B, it is assumed that the terminal 421 fails in decoding the first data packet. Thereafter, the terminal 421 receives scheduling information from the serving base station 411 (Cell-0) in a subframe 3 to receive the first HARQ packet from the base station 413 (Cell-5) indicated by the scheduling information. Referring to (C) of FIG. 4B, it is assumed that the terminal 421 fails in decoding the first HARQ packet received from the base station 413 (Cell-5). Thereafter, the terminal 421 receives scheduling information from the serving base station 411 (Cell-0) in a subframe 7 to receive the second HARQ packet from the base station 417 (Cell-7) indicated by the scheduling information.
As shown in FIGS. 4A and 4B, if a particular base station of a serving base station group is designated as a base station for transmitting and receiving a scheduling and/or control signal for a corresponding terminal, for an unstable link between the base station and the terminal, transmission and reception of the scheduling and/or control signal may be difficult to perform, hindering data communication from being continued. Even when a channel condition of another base station of the serving base station group is better than the serving base station for transmitting the scheduling and/or control signal, the scheduling and/or control signal needs to be transmitted and received through the serving base station, disturbing optimization of performance of a channel for transmitting and receiving the scheduling and/or control signal.